1. Field of the Invention
The present invention relates to a touch sensor board, an image display device, and a manufacturing method of the touch sensor board. More specifically, the present invention relates to a touch sensor board and the like with which high screen visibility can be achieved and manufacture failure does not occur so easily.
2. Description of the Related Art
Recently, electronic apparatuses such as smartphones, tablets, and notebook personal computers provided with a touch panel formed by combining a liquid crystal display with a touch sensor board as an input/output module have become common. With such touch panel, input operations can be done by touching the display screen directly with a finger (or a touch pen, or the like). Therefore, it is possible to achieve a user interface that can be operated by a user intuitively and conveniently.
Among the touch sensor boards, particularly a projected capacitive type (referred to as a PCAP type hereinafter) is used most frequently for such touch panels. This is a type which detects changes in the static capacitance of a part touched by the user with sensor electrodes extendedly provided in the longitudinal and lateral directions on the touch sensor board. The touch sensor board is formed with a plurality of sensor electrodes constituted with first electrodes and second electrodes being insulated electrically and an interlayer film provided for achieving interlayer isolation (insulation) at least at the intersections between the first electrodes and the second electrodes.
With this system, it is possible to detect the position of a finger when the finger simply approaches a sensor electrode even though the finger does not touch the sensor electrode directly. This makes it possible to place a cover such as glass on the surface of a touch panel, so that this system is excellent in terms of durability, environment resistance, design, and the like. Further, this system also is high in detection accuracy of the positions at which the finger touches, and a greater number of points can be detected. Therefore, it is possible to perform complicated input operations such as slide input, flick input, gesture input, and the like with smartphones, thereby making it possible to greatly contribute to improving the operability.
On the other hand, the touch sensor board is required to form the sensor electrodes and the interlayer film with a colorless transparent material so that the visibility of the liquid crystal display is not obstructed. It is common to use ITO (indium tin oxide) as the material for the sensor electrodes and to use an acryl resin or a polyimide resin as the material for the interlayer film. However, none of those is completely colorless and transparent.
Thus, deterioration in the visibility of the display screen caused due to the optical characteristic of the sensor electrodes and the interlayer film is generated. In order to improve that point, many technical experiments have been tried since. Hereinafter, the notable experiments among those will be described.
Basically, as the structure of a touch sensor board, there are a case where the above-described first electrodes and second electrodes are formed separately and a case where those are formed with a same single layer. Out of those cases, the sensor electrodes are formed with separate layers by sandwiching an interlayer insulating film therebetween with the structure where the electrodes are formed with two layers. Thus, the optical path length for the reflection light varies between the first electrodes and the second electrodes, which causes difference in the optical characteristics so that the visibility is deteriorated.
Japanese Unexamined Patent Publication 2009-265748 (Patent Document 1) discloses a technique which forms the first electrode and the second electrode of the sensor electrodes with a same single layer. By forming the first electrode and the second electrode with the same single layer, the optical characteristics becomes identical at least in the areas where the sensor electrodes exist, so that the deterioration of the visibility can be suppressed.
Japanese Unexamined Patent Publication 2010-140370 (Patent Document 2) and JP 4720857 B (Patent Document 3) disclose a technique which forms a minimum necessary interlayer film pattern only in the intersections between connection patterns connected electrically to the first electrodes and the second electrodes, respectively, so as to suppress deterioration of the transmittance thereby. With a structure where the interlayer film exists on the entire surface, the transmittance is deteriorated in no small quantities due to the existence of the interlayer film. Especially in a case where the interlayer film is formed in film thickness of an organic resin or the like for decreasing the coupling capacitance between the first electrode and the second electrode, the transmittance is deteriorated more prominently. Therefore, it is possible to suppress deterioration in the transmittance through forming the interlayer film only in the intersections.
Patent Document 3 further discloses a structure which forms a dummy pattern having a refractive index equivalent to that of an electrode pattern in the area with no electrode pattern, i.e., in a part of the gap between the neighboring electrode patterns, so as to suppress deterioration in the visibility caused due to the difference between the optical characteristics of the section having the sensor electrode and the section with no sensor electrode in order to improve the visibility.
However, it is necessary with this technique to secure the space for both the dummy pattern and the electrode pattern when the dummy pattern is formed simultaneously with the electrode pattern, so that the section with no electrode pattern cannot be eliminated completely. Thus, the effect of improving the visibility is small. Therefore, in Patent Document 3, it is also disclosed to form the dummy pattern with a different material from that of the electrode pattern.
The techniques depicted in each of Patent Documents 1 to 3 described above are named as existing techniques 1 to 3, respectively, and those will be described in the following paragraphs.
(Regarding Existing Technique 1)
FIG. 16 is a plan view showing the structure of a touch sensor board 910 (according to Existing Technique 1) depicted in Patent Document 1. The touch sensor board 910 is of a PCAP type, in which first electrodes 911 and second electrodes 912 as the sensor electrodes are formed on a transparent board 916 to be neighboring to each other on a same plane with a same single layer. The first electrodes 911 are extended by being electrically connected via same-layer connection patterns 913, while the second electrodes 912 are extended in a different direction from that of the first electrodes by being electrically connected via different-layer connection patterns 914 disposed on a different layer from the second electrodes 912.
Note here that the same-layer connection pattern 913 and the different-layer connection pattern 914 are formed to be electrically interlayer-insulated via an interlayer film 915. At the same time, the interlayer film 915 is formed to become an isolated pattern by including the intersection area described above and to have no overlapping part with the second electrodes 912.
FIG. 17 is a sectional view taken along a line H-H′ of FIG. 16. The touch sensor board 910 is fabricated via each of steps in which: the different-layer connection patterns 914 are first formed on the transparent board 916 and the interlayer film 915 is formed successively in this order; the first electrodes 911, the second electrodes 912, and the same-layer connection patterns 913 are formed on a same layer (this layer is called a sensor electrode layer); and a protection layer 917 is formed on the sensor electrode layer at last.
With this structure, an exposed region 918 that does not overlap with either the pattern of the sensor electrode layer or the pattern of the interlayer film 915 exists. The exposed region 918 is exposed at the time of etching processing performed when forming the pattern of the sensor electrode layer. Thus, as a film material for the sensor electrode layer and a film material for the different-layer connection patterns 914, it is required to select materials that are selective for the etching processing of the sensor electrode layer. That is, the film materials are limited, so that it is not possible to select the optimum film materials by considering the transmittance of light, electric resistance, and the like.
(Regarding Existing Technique 2)
As a technique for overcoming the above-described issue of Existing Technique 1, there is Existing Technique 2. FIG. 18 is a plan view showing the structure of a touch sensor board 920 (according to Existing Technique 2) depicted in Patent Document 2.
The touch sensor board 920 is of a PCAP type, in which first electrodes 921 and second electrodes 922 as the sensor electrodes are formed on a transparent board 926 to be neighboring to each other on a same plane with a same single layer. The first electrodes 921 are extended by being electrically connected via same-layer connection patterns 923 disposed on a same layer as the first electrodes 921, while the second electrodes 922 are extended in a different direction from that of the first electrodes by being electrically connected via different-layer connection patterns 924 disposed on a different layer from the second electrodes 922.
Note here that the section where the same-layer connection pattern 923 and the different-layer connection pattern 924 intersect with each other is formed to intersect by being electrically inter-layer insulated via an interlayer film 925. The interlayer film 925 is formed limitedly in the intersections between the same-layer connection patterns 923 and the different-layer connection patterns 924, and a part thereof is formed to overlap with the second electrodes 922.
Therefore, the different-layer connection pattern 924 necessarily overlaps with the pattern of the sensor electrode layer or the interlayer film 925, so that there is no exposed part generated at the time of the etching processing like the exposed region 918 shown in FIGS. 16 and 17. Thus, it becomes unnecessary to select the film material exhibiting selectivity for the etching processing, so that the optimum film materials can be selected.
That is, the issue of Existing Technique 1 described above can be overcome. However, there is another issue generated thereby. This point will be described.
FIG. 19 is a perspective view showing an enlarged view of an end vicinity region 923a of the same-layer connection pattern 923 of the touch sensor board 920 shown in FIG. 18. As in the case of Existing Technique 1, manufacturing steps of the touch sensor board 920 are as follows. That is, the different-layer connection patterns 924 are first formed on the transparent board 926 and then the interlayer film 925 is formed successively in this order; and the sensor electrode layer is formed thereafter.
Note here that the first electrodes 921, the second electrodes 922, and the same-layer connection patterns 923 as the sensor electrode layer are formed after the interlayer film 925 is formed to be an isolated pattern. Regarding the same-layer connection patterns 923 and the different-layer connection patterns 924, it is necessary to set the constant at the time of wiring the extended sensor electrodes through minimizing the parasitic capacitance formed between those connection patterns. Thus, the interlayer film 925 is formed in relatively thick film thickness.
Due to steps generated by the film thickness, there is a remaining film 928 of the sensor electrode material generated along the end part of the interlayer film 925. This causes an issue of having short-circuit generated between the neighboring first electrodes 921 and second electrodes 922.
In the manufacturing steps, the sensor electrode material is deposited on the board where deposition of the interlayer film 925 has been completed, and a photoresist for forming the patterns of the first electrodes 921 and the second electrodes 922 is applied. At that time, the photoresist is applied thicker in the vicinity of the large step of the interlayer film 925 than in the other areas, thereby deteriorating the exposure and development etching characteristic. As a result, the remaining film of the photoresist is generated along the end part of the interlayer film 925. This remaining film is the cause for generating the remaining film 928 of the sensor electrode material, which forms the short-circuit path between the neighboring sensor electrode patterns.
(Regarding Existing Technique 3)
As a technique for overcoming the above-described issue of Existing Technique 2, there is Existing Technique 3. FIG. 20 is a plan view showing the structure of the touch sensor board 930 (according to Existing Technique 3) depicted in Patent Document 3.
The touch sensor board 930 is of a PCAP type, in which first electrodes 931 and second electrodes 932 as the sensor electrodes are formed on a transparent board 936 to be neighboring to each other on a same plane with a same single layer. The first electrodes 931 are extended by being electrically connected via same-layer connection patterns 933 disposed on a same layer as the first electrodes 931, while the second electrodes 932 are extended in a different direction from that of the first electrode by being electrically connected via different-layer connection patterns 934 disposed on a different layer from the second electrodes 932.
Note here that the same-layer connection patterns 933 and the different-layer connection patterns 934 are formed to be electrically interlayer-insulated via an interlayer film 935. At the same time, the interlayer film 935 is not formed to be an isolated pattern but formed almost over the whole surface on the touch sensor board 930 except for through holes 935a opened in the connection parts between the second electrodes 932 and the different-layer connection patterns 934.
Thus, the different-layer connection patterns 934 overlap with either the pattern of the sensor electrode layer or the interlayer film 935, so that there is no exposed part generated at the time of performing the etching processing like the exposed region 918 shown in FIGS. 16 and 17. Therefore, the issue of Existing Technique 1 does not occur. Further, Existing Technique 3 has no pattern end that may possibly form the short-circuit path between the neighboring sensor electrode patterns as in Existing Technique 2. Therefore, the issue of Existing Technique 2 does not occur.
However, Existing Technique 3 faces another issue generated because the interlayer film 935 is formed almost over the entire surface. This issue may possibly occur with the structures where the interlayer film is formed only in the intersections between the sensor electrodes as in Existing Techniques 1 and 2, i.e., with the structure where the interlayer film is not formed almost on the entire surface. This will be described.
FIG. 21 is a sectional view taken along a line I-I′ of FIG. 18 (Existing Technique 2). On the touch sensor board 920 according to Existing Technique 2, the interlayer film 925 is formed only in the intersections between the same-layer connection patterns 923 and the different-layer connection patterns 924. Thus, as the paths where the light of screen display transmits through, there are two kinds of paths such as a path 3A “the transparent board 926→the first electrode 921 or the second electrode 922→the protection film 927” and a path 3B “the transparent board 926→the protection film 927”.
There is a difference generated in the transmittance as the entire touch sensor board 920 between the path 3A that passes through the sensor electrodes and the path 3B that does not pass through the sensor electrodes for the amount of the transmittance of the sensor electrodes (the first electrodes 921 or the second electrodes 922). Note here that the number of films the light has to passes through is different for the paths 3A and 3B. That is, the number of film interfaces is different, so that it is difficult to control the reflectance. This causes difference in the reflection characteristics between those paths.
The difference in the reflection characteristics provides a state where the user can recognize the electrode patterns visually at least with a specific display luminance and a specific viewing angle. This causes the critical deterioration in the display quality.
FIG. 22 is a sectional view taken along a line J-J′ of FIG. 20 (Existing Technique 3). In the touch sensor board 930 according to Existing Technique 3, as the paths where the light of screen display transmits through, there are two kinds of paths such as a path 4A “the transparent board 936→the interlayer film 935→the first electrode 931 or the second electrode 932→the protection film 937” and a path 4B “the transparent board 936→the interlayer film 935→the protection film 937”.
That is, in the case of the touch sensor board 930 according to Existing Technique 3, the film interfaces are to increase for the amount of the transmittance of the interlayer film 935 formed almost on the entire surface. Thus, compared to the case of Existing Technique 2, control of the reflectance becomes more difficult. In Patent Document 3, it is also depicted as a countermeasure for such issue to “form a dummy pattern as another layer with a material different from that of the electrode patterns”. However, this evidently complicates the manufacturing steps and causes a great increase in the cost, so that it is not practical.
(Summary of Issues Regarding Existing Techniques)
As the summary of the above, the issues regarding the PCAP-type touch sensor board in Existing Techniques are three points in the followings.
(Issue 1)
Due to the existence of the exposed region 918 exposed at the time of the etching processing performed when forming the patterns of the sensor electrode layer, selection of the optimum film material becomes difficult. Accordingly, the manufacture readiness is deteriorated and the cost is increased (Existing Technique 1).(Issue 2)The remaining film of the photoresist tends to be generated in the isolated interlayer film end part. This causes short-circuit failure between the sensor electrodes, thereby deteriorating the manufacture yield (Existing Technique 2).(Issue 3)There is a difference generated in the transmittance as the entire touch sensor board between the path that passes through the sensor electrodes and the path that does not pass through the sensor electrodes for the amount of the transmittance of the sensor electrodes, so that control of the optical characteristic becomes difficult. Therefore, it is likely to deteriorate the display quality, e.g., the user can recognize the electrode patterns visually (Existing Techniques 1, 2, 3).
As described above, with Existing Techniques 1 to 3, the structure for overcoming one issue may be the factor for causing another issue such as the cases of Issue 1 and Issue 2 regarding the structure of the interlayer film. Further, as in the case of Issue 3, another intrinsic issue stands as the issue that cannot be overcome.
It is therefore an exemplary object of the present invention to overcome Issues 1 to 3 described above simultaneously and to provide the touch sensor board, the image display device, and the manufacturing method of the touch sensor board capable of improving the visibility of the screen and the manufacture yield (manufacture cost) simultaneously.